Many collectors are working through levelling their pets, and it can be hard to keep track of hundreds of less-than max level pets.
As such, I'd like to see a new filter added - being able to include or exclude max (currently 25) level pets.
That way, if you just want to (for example) see what lvl 25 mechanicals you have available to take out that annoying beast that's bugging you, that's easy to do (just filter collected, including max level and including mechanical).
And if you want to see which rare, non-duplicate aquatic pets you have to level (for example), you can also easily do that by simply combining filters (in this case include collected (rare), exclude collected (duplicates), include "can battle", include "aquatic" and exclude "max level".
The max level filter naturally wouldn't be able to catch non-battling pets since they don't have levels.